In the existing combined systems having ion columns (FIB) and electron columns (SEM) the space near the sample is very limited, since, apart from electron and ion columns, detectors for different types of signals, such as secondary electron detectors (SE), backscattered electron detectors (BSE), secondary ion detectors (SI), cathodoluminescence detectors (CL), Auger electron detectors, X-ray analyzers, such as energy dispersion spectroscope (EDS), wavelength-dispersive spectroscope (WDS), and analyzers of electron backscatter diffraction (EBSD) are installed in the vacuum chamber. Combined systems may potentially include optical microscopes, laser beams, scanning probe microscopes (e.g., atomic force microscopy AFM), gas injection systems (GIS), sample heating and cooling, manipulators, and other equipment. Usually, the closer the mentioned analyzers are to the sample, the better is the collection angle, and the optimum signal is often perpendicular to the sample, resulting in a “fight for space” and many compromises made when designing more complex devices provided with a large number of detectors and analyzers.
Further limitation of space above the sample occurs in relation to a requirement for the shortest working distance (i.e., a distance between the column pole tip and the sample). For example, the shorter working distance of electron microscopes, the better the resolution; therefore, the working distance is 1 mm for imaging and 5-15 mm for analysis is usually used. Similarly, when working with higher pressure in the chamber than in the column, for example in environmental microscopy, the shortest possible working distance is required in order to maintain the quality of a primary electron beam during its travel through higher pressure in the chamber.
The above mentioned factors result in a lack of space for placing the signal detectors. In particular, the conventional detector BSE, which is usually concentric around the electron column optical axis with the width of more than 10 mm and closely above the sample practically prevents using other necessary detectors or considerably reduces the effectiveness of collecting the other signals. A typical solution is a BSE detector retractable from a side (i.e., perpendicular to the electron column optical axis), and is retracted when not collecting a signal. However, it is often necessary to read signals simultaneously from several detectors including the BSE. Furthermore, the conventional BSE detector limits the sample tilt required for example for working using a FIB device, when the sample needs to be tilted perpendicular to the ion column axis. In order to allow such tilt of the sample, the detector area is often cut, or eventually the grooves are cut, or holes for other detectors and analyzers are drilled. However, this results in a smaller detection area, lower quality of detected signal and broken detection symmetry (i.e., homogeneity of the signal).
A wide range of technological solutions protected by patents try to deal with the mentioned spatial problems concerning the signal detection. Based on the U.S. Pat. No. 5,387,793, an SE detector is placed above the objective lens in electron microscope, wherein using additional electrodes creating crossed magnetic and electrostatic fields (also known as E×B or so-called Wien filter) which point SE to the SE detector, not affecting the primary electron beam. Based on the U.S. Pat. No. 5,578,822, the device combines a conventional lens with an immersion single-pole lens arranged below. A secondary electron detector is placed in between of these lenses. Other solutions, such as U.S. Pat. No. 6,600,156 and CZ 298912, move the SE detector into the electron column in the area of the objective lens, where holes are drilled from a side to detect signal electrons.
The mentioned solutions demand space in the electron column. In addition, the mentioned drilling in the objective lens causes problems with column sealing for environmental microscopy and problems related to weakening of the material in the magnetic circuit. Weakening of the material leads to magnetic saturation and undesirable extension of magnetic field to the optical axis. Moreover, installation of other elements creating crossed E×B fields occupies further precious space in the electron column. Therefore, some other necessary elements, such as scanning coils, are limited. In addition, current trends include SEM systems with number of different detectors to collect different signals emitted from the sample under different angles. For example, preferred configuration of the electron microscope comprises a single BSE detector arranged closer to the sample, which collects high-angle signal electrons, and another BSE detector further from the sample, which collects signal electrons closer to the electron column. In a structure of an electron column with two or three BSE detectors and further two or three SE detectors, the space in the electron column is unacceptably over-filled.
In case of electron columns with an inserted isolated tube on high positive potential, see e.g. U.S. Pat. No. 7,425,701 or US 2014/0361167, the need to integrate SE and BSE detectors into electron column leads to special solutions which are complicated in terms of structure. For example, the solution for detection in a combined system on page 5 of ZEISS Crossbeam Family document, Your FIB-SEM for High Throughput 3D Analysis and Sample Preparation, 14 Mar. 2017, https://applications.zeiss.com/C125792900358A3F/0/FCB253FF6416DBE8C1257BD00048BC58/$FILE/EN_42_011_091_Crossbeam_rel2_0.pdf with an electron column with a potential tube is based on a placement of signal electron detectors inside the electrostatic field from the potential tube. Both SE detector and BSE detector are arranged therein. In general, adding another detector into the electron column is complicated in such systems and, as a consequence, the structure of the whole electron column is changed.
By combining the requirements for a large number of devices in the chamber in close vicinity to the sample, the shortest working distance possible, and problematic arrangement of detectors directly into the electron column, results in a problem with suitable placement of the signal electron detectors. Furthermore, in combined FIB-SEM systems, in contrast to a stand-alone SEM, there is an option to detect secondary ions (SI) and ion-induced secondary electrons (ISE). When working with ion beam in a combined FIB-SEM system, the sample is usually tilted perpendicularly to the FIB column. Ion and electron column in these devices usually form 40-70° or 90° angle, and thus the detectors placed in the electron column collect only a part of the signal traveling under relatively high angle from the perpendicular line to the surface of the sample. However, the highest signal concentration is along the perpendicular line towards the surface of the sample, therefore, only a small part of the signal is detected.